The present invention relates to turbines, e.g., gas turbines, and particularly to apparatus and methods for welding turbine vane segments one to the other to form an annular array of segments forming a turbine stage.
In the construction of turbines, particularly gas turbines, an annular array of turbine segments is provided to form a turbine stage. Generally, the turbine stage includes outer and inner annular bands spaced radially one from the other with a plurality of vanes extending between the outer and inner bands at circumferentially spaced positions one from the other. It will be appreciated that the outer and inner bands and the vanes define a path for the working fluid flowing through the turbine, e.g., a hot gas path in the case of a gas turbine. In a more recent advanced gas turbine design, the hot gas path is cooled by flowing steam along walls of the outer and inner bands and through the vanes. For various reasons, including the complexities of cooling a gas turbine with steam, the nozzle stages are formed of discrete segments or singlets. Each singlet or vane segment, because of the airfoil shape of the vanes, includes forward, generally axially extending outer and inner band portions, and aft outer and inner band portions which extend both axially and circumferentially, i.e., angled in an aft direction relative to the axis of the rotor and relative to the forward portions of the segment.
The singlets are preferably welded together along adjoining margins of the outer and inner bands. However, the materials forming the outer and inner bands are necessarily designed for high strength and resistance to the high temperatures of the hot gas path. The inner and outer bands are therefore not readily welded to one another because of their necessary chemistry, conditioning and configuration. While tungsten inert gas (TIG) welding techniques are common, they are characterized by uneven heat input to the weld, resulting in significant warpage and distortion of the parts welded to one another. The filler material must also characteristically match the material of the outer and inner bands. Accordingly, a stator vane assembly and method of forming the assembly are required which eliminate or minimize any distortion in the welds and which provide a high strength joint.
In accordance with a preferred embodiment of the present invention, the adjacent margins of the outer and inner bands of adjoining stator vane segments are provided with generally radially outwardly and inwardly directed flanges, respectively. Chamfers are also formed along their adjoining edges on the hot gas path sides. By employing E-beam welding, the material of the margins, i.e., the flanges, are fused one to the other using the parent material and without the addition of high heat inputs characteristic of other welding techniques such as TIG welding. The resulting weld is without significant distortion or warp and has significantly fewer defects than TIG welds. While E-beam welding is not the most cost-effective type of welding, e.g., it must be performed in a vacuum, it has the advantage of significantly eliminating or minimizing distortion or warpage because of its reduced heat input. By fusing the margins of the outer and inner bands to one another by E-beam welding with the E-beam directed from outside of the outer and inner bands, it will be appreciated that the parent material may spatter weld material on the gas path side and form an irregular surface. Consequently, chamfers are provided on the margins of the outer and inner bands along the hot gas path sides thereof such that, subsequent to welding the outer and inner margins together using E-beam welding, the interior chamfers are TIG-welded. The weld material added to the mating chamfers and the higher heat input relative to E-beam welding does not affect the joint because the weld area is significantly smaller than welding the entirety of the margins to one another. The weld material applied along the chamfered surfaces of the outer and inner bands by TIG-welding can be machined to form a smooth, continuous gas path surface along the hot gas path side of the outer and inner bands.
At the aft end of the outer band of each segment, there is provided an aft hook for securing the nozzle stage to the fixed casing of the turbine. The aft hook comprises a radially outwardly directed flange and a flange which projects in an aft direction from the radial flange. These flanges change the thickness of the material forming the margins of the outer band at the aft hook. E-beam welding requires a constant energy input and the material being welded should have a constant cross-sectional geometry to effect a uniform weld. The aft hook changes that geometry and therefore the aft hooks of the adjoining segments are not suitable for welding using solely an E-beam. To accommodate the change in material thickness, and still provide a high-strength weld without distortion or warpage, the adjoining ends of the aft hook at the juncture of the vane segments are cut back to provide angled faces spaced circumferentially one from the other and inset in a circumferential direction from the segment margin. A filler piece is provided for disposition in the slot thus formed between the inset faces of the aft hook portions of the vane segments. The filler piece has a reduced thickness tongue projecting in the same angled direction as the aft projecting flange. The cutout or slot in the adjoining aft hook segments are provided with a larger spacing between the inset faces of the aft projecting flange than between the inset faces of the radial projecting flange. Hence, the reduced tongue of the filler piece provides gaps between it and the inset faces of the aft projecting flanges of the adjoining segments. The body of the filler piece is preferably E-beam welded to the radial projecting flange faces and TIG-welded to the aft projecting flange faces. Weld material is therefore supplied in the gaps between the tongue and the inset faces of the aft projecting flange. By employing these welding techniques at the various locations of the adjoining segments, the individual vane segments are joined one to the other without substantial warpage or distortion.
In a preferred embodiment according to the present invention, there is provided a method of welding first and second stator vane segments to one another wherein each segment includes an outer band, an inner band and a vane extending between the bands comprising the steps of welding adjacent margins of the inner and outer bands of the first and second segments to one another from outside the bands without using weld filler material and welding the adjacent margins to one another along inside surfaces of the bands using weld filler material.
In a further preferred embodiment according to the present invention, there is provided a method of welding first and second stator vane segments to one another wherein each segment includes an outer band, an inner band and a vane extending between the bands comprising the steps of welding adjacent margins of the inner and outer bands of the first and second segments to one another, providing an opening along aft hooks of the outer bands defining opposed setback faces, inserting a filler piece in the opening spaced from the setback faces and welding the filler piece to the aft hooks.
In a still further preferred embodiment according to the present invention, there is provided a stator vane assembly for a turbine comprising first and second stator vane segments each including an outer band, an inner band and a vane extending between the outer band and the inner band, the outer and inner bands of the first segment having respective first margins welded to second margins of the outer and inner bands, respectively, of the second segment whereby the first and second segments are secured to one another to define a flow path between adjoining vanes and the inner and outer bands of the segments, each of the segments having an aft hook along an aft edge of the outer band thereof and having a generally radially extending flange and an aft projecting flange, the flanges of the aft hooks being set back from respective first and second margins of the segments forming setback faces defining an opening between the flanges, a filler piece in the opening defining gaps on opposite sides thereof with the setback faces of the aft projecting flanges and weld material in the gaps securing the filler piece and the aft projecting flanges to one another.